Mantle xenoliths carried by kimberlite magma provide the only hard evidence of the rock types that constitute cratonic roots and the conditions under which they formed. Here, we present the first such data for the Slave craton (northern Canada) based on samples of peridotite, eclogite, and pyroxenite collected from a Middle Jurassic kimberlite (Jericho pipe). The petrology and thermobarometric analysis of these mantle xenoliths provide estimates of the thermal state and stratigraphy of the underlying mantle and constrain the evolution of this ancient craton. The upper mantle beneath the north Central Slave is relatively cold, as is typical of cratonic environments, but it is also associated with relatively high values of surface heat flow (47–50 mW/m2) because of this craton's highly radiogenic crust. Below 160–190 km the Jericho P-T array shows a significant high-temperature disturbance caused by transient magmatic events. The apparent source region for both porphyroclastic peridotite and other texturally unequilibrated magmatic rocks coincides with this thermal excursion and is taken to mark the base of the petrological lithosphere. The depth of this petrological transition is also consistent with important geophysical discontinuities mapped below the Slave craton.